Holistic Evaluation of a Representative Link – Development of a Water Availability & Simulation Model

Abstract

Any plan related to inter-basin transfer of water from a water-surplus basin to a water-deficit basin has to take into account the water availability and demands under the present and future scenarios of water use. Any water-related activity that takes place in one part of a river basin may have consequences in the other part. Therefore, effective management of water and related environment in a river basin requires an integrated and coordinated planning within the basin. In the present approach of water availability estimation in a river basin, it is very difficult to account for the effect of land use change and climate change on the water resources scenario. Water requirement for different purposes (nature, food, and people) is not precisely estimated. Discharge is considered as the basic unit for water availability estimations which may be affected by a number of basin parameters and variables such as population, industrialization, change in the irrigated areas, improvement in irrigation efficiencies, availability and development of groundwater, change in land use (increase/ decrease in forest area, urban land, barren land etc.), change in the climate of the region (increase/decrease in temperature, precipitation etc.), construction of hydraulic structures etc. Therefore, a need was felt to develop a detailed model to assess the available water resources in a river basin and to estimate the demands for various purposes. A conceptual spatially distributed water balance model has been developed to simulate various components of the hydrologic cycle at the scale of a river basin. Various types of spatial, attribute, and dynamic data are integrated by the model to perform the water balance analysis of a basin. Spatially distributed information include land-use map, crop map, soil map, Thiessen polygon map of rainfall and ET stations, district boundary map, map showing major cities in the basin, digital elevation map, slope map, flow direction map, groundwater depth map, river network map, storage/diversion structure map, irrigation source map, initial moisture map, and aquifer characteristics (storage coefficient and transmissivity) maps. Attribute information contains the properties and general information about to different types of crops, soils, hydraulic structures, river network, gauging sites, and domestic demand standards in the basin. Dynamic information of the basin include daily rainfall and reference evapotranspiration at different observation stations, observed monthly water flows at various gauging sites and groundwater levels in different observation wells in the basin. The model computes various components of hydrologic cycle such as rainfall, evapo-transpiration, runoff, groundwater recharge, soil moisture change etc. for various land uses in different sub-basins of a river basin. The model brings out total water availability in the basin; water consumed by different uses; and water storage in different hydraulic structures, in soil water zone, and in groundwater aquifer in a river basin. Using this model, various scenarios of water availability in a river basin can be generated and analyzed. By taking repeated runs of the model for longer time periods, sustainability of various water resources management plans can be examined. Further, the effect of various factors such as: i) change in landuse (increase or decrease in forest area, cultivated area, barren land etc.), ii) change in the cropping pattern in the area, iii) change in water use and conveyance efficiencies, iv) construction of new water resources projects/change in the design of existing projects, v) change in population and corresponding D&I demands on the basin water resources can be analyzed with the model. The model has been applied to the Tapi basin up to Ukai dam. Extensive database has been generated for the basin and model runs have been taken from June 1992 to May 1996. Basin data has been used to check the model linkages. Various outputs of the model for the Tapi basin have been discussed in detail.